Soundly asleep in their beds, the residents of Yucca Valley and
Big Bear Lake were not the only ones jolted by last month's early
morning earthquakes. Scientists met with surprises, too.

"When I drove down there, I fully expected to find that a major
east-west fault line called the Pinto Mountain fault had
ruptured," says LBL geologist Pat Williams, who has been studying
ancient fracture patterns along the Hayward fault (see
Currents, June 29, 1990). Instead, the largest earthquake
to occur in the contiguous 48 states in the last 40 years broke
through four separate, lesser-known north-south fault lines in
the Mojave Desert. "It let me know again that the Earth will
always surprise you," Williams says.

The magnitude 7.4 Yucca Valley quake, named the Landers
earthquake for the nearest, hardest-hit town, ruptured at 5 a.m.
on June 28. A second one, 6.5 on the Richter scale, hit about
three hours later, 30 kilometers west at Big Bear Lake, a ski
resort in the San Bernardino Mountains. News reports that day
said the two quakes were "unrelated," but reports the next day
said they were related. Williams explains that although
technically speaking, the second may have been too far away to be
called an aftershock, the two earthquakes were clearly related in
that the Landers earthquake lowered the compressional stress on
the Big Bear fault and allowed it to rupture.

It also lowered the compressional stress across the central
portion of the lower San Andreas fault. "It didn't go off, but
the period of worry is not over," says Williams, who believes
that the southern end will be the next big section of the San
Andreas to rupture. He was concerned that slippage on the four
Yucca Valley faults would set up a continuation north toward
Barstow and south to the San Andreas via the Joshua Tree
structure that slipped in April. If the San Andreas fault were to
break as far north as the Cajon Pass (where Interstate 15 pierces
the San Bernardino Mountains to connect Palmdale and San
Bernardino) and south to the Salton Sea (which lies south of the
Mojave Desert), Williams predicts the quake would be a magnitude
8.0 -- the Big One the experts are expecting.

The Landers earthquake is providing scientists with a wealth of
information. "The Yucca Valley zone is very wide and complex,
with lots of large and lots of small displacements," Williams
says.

The fault rupture that produced the earthquake had little trouble
stepping across the gaps that separated its four separate fault
lines -- the Johnson Valley, Homestead, Emerson, and Camp Rock
faults. Williams says in the past similar steps in many "dormant"
faults have been considered probable barriers to slip. For
example, a step near San Leandro has been considered a barrier to
the rupture of the full length of the Hayward fault. "This
conclusion," he says, "will now be viewed with added caution."

Williams and Earth Sciences technician Preston Holland flew to
the area the day of the quake, rented a car, and crisscrossed the
fault zone, providing the U.S. Geological Survey with
reconnaissance mapping of a 29-kilometer area -- or about a
quarter of the fault zone -- to guide their interpretation of
aerial photographs. Often, the LBL geologists found two major
boundary fractures, between 10 and 30 meters apart, with many
diagonal faults in between.

"This is something many of us have seen before, but never on such
a large scale," Williams says. "We'll learn a lot about the
possible range of surface patterns, which should have application
to the zoning of faults."

Holland was struck by the similarity between a fault structure he
saw near Landers -- in which the fault takes a side-step to
create a pull-apart depression and a compression ridge -- and an
ancient structure revealed by a trench in Fremont where the LBL
geologists are studying the Hayward fault.

"To see a surface break through that kind of feature gives us a
better idea of what we might be looking at in the trenches,"
Holland says. "We've already started to interpret some of the
features we see in the trenches a little differently based on
what we saw at Landers."

A theory that was posed in the aftermath of last month's
earthquakes was that a new San Andreas fault could be opening up
from the Salton Sea north to central California. Williams says
instead that the recent activity has linked the Gulf of
California with slip systems east of Mount Whitney, the 1872 site
of one of California's three historic magnitude 8.0 earthquakes.

Williams' theory is that the strain is heading toward the back
side of the Sierras, northeast into central Nevada, where it may
affect active geothermal fields. The earth's movement could
enhance the production of geothermal fluids by contributing to a
thinning of the crust and allowing the fluids to circulate
through complex fractures.

A huge strain response on a scale never seen before occurred
during the 24 hours following the Landers earthquake. At the
Pinion Flats Observatory researchers from UC San Diego observed a
massive redistribution of strain deep in the earth's crust. For
the first time, Williams says, scientists will be able to study
not just how the upper 12 kilometers of brittle crust reacts, but
the response of the deep crust beneath it.

Seismic activity in California continues at a pace not unexpected
in the aftermath of such major shocks. Compared to the average
yearly rate of two magnitude 5.0 earthquakes in southern
California, there have already been at least 10 since June 28.
One that has caused some concern occurred a few days afterward
near Lathrop Wells on U.S. 95, about 10 miles from the potential
nuclear waste burial site at Yucca Mountain. Another 5.0 quake
just this Saturday shook the large east-west Garlock fault, which
forms the Mojave's northern boundary.